Communication system receivers often employ image rejection to remove undesired information from received signals. The term "image rejection" is well known in the art to refer to the removal of certain undesirable information, such as a particular frequency band. In the context of wireless telephony, an example of undesired information is a conversation carried on another channel.
Image rejection in receivers has traditionally been provided by passive filters. More recently, image rejection is being implemented in receiver down converters to ease the rejection requirements of preceding passive filters. As is well known in the art, receiver down converters process a received signal and generate a down-converted signal having a lower frequency. A conventional receiver down converter is described with reference to FIGS. 1, 2A and 2B.
FIG. 1 is a block diagram illustrating a conventional image reject down converter arrangement 100. A received signal s(t).sub.1 is processed by a passive filter 102 that removes undesirable information from signal s(t).sub.1 and provides a filtered signal s(t).sub.1 '. One example of a passive filter that is typically used for passive filter 102 is a surface acoustic wave (SAW) filter. Filtered signal s(t).sub.1 ' is provided to an automatic gain controller (AGC) 106 that adjusts the gain of signal s(t)' to provide an output signal s(t).sub.1 ". The gain of signal s(t).sub.1 " provided by AGC 106 is selected to be compatible with, i.e. not saturate, a band-pass (BP) filter 122 that is described in more detail hereinafter.
Signal s(t).sub.1 " is provided to an image reject mixer 107. Within image reject mixer 107, signal s(t).sub.1 " is provided to a mixer 108 along with a signal s.sub.LO generated by a local oscillator 110 and having a frequency of f.sub.LO. Mixer 108 generates a signal V.sub.I '. Signal s.sub.LO is also provided to a phase shifter 112 that generates a phase shifted signal s.sub.LO ' that is ninety (90) degrees out of phase with respect to signal s.sub.LO. Multiple phase shifters are sometimes employed to provide the signals s.sub.LO and s.sub.LO ' so long as the total phase shift between signals s.sub.LO and s.sub.LO ' is ninety (90) degrees. For example, two phase shifters providing phase shifts of plus forty five (+45) degrees and minus forty five (-45) degrees may be employed.
A mixer 114 combines signal s(t).sub.1 " with phase shifted signal s.sub.LO ' and generates a signal V.sub.Q '. Signal V.sub.I ' is ninety (90) degrees out of phase with respect to signal V.sub.Q '. Signal V.sub.I ' is processed by a phase shifter 116 that generates a signal V.sub.I that is forty five (45) degrees out of phase with respect to signal V.sub.I '. A phase shifter 118 processes signal V.sub.Q ' and provides a signal V.sub.Q that is one hundred thirty-five (135) degrees out of phase with respect to signal V.sub.Q '. Thus, phase shifters 116 and 118 provide an additional ninety (90) degrees of phase shift of the unwanted image information contained in signals V.sub.I and V.sub.Q.
Signals V.sub.I and V.sub.Q are provided to a summer/subtractor 120 that selects either an upper or lower side band to be removed. Summer/subtractor 120 provides a down-mixed signal V.sub.IF having a center frequency of f.sub.IF. Signal V.sub.IF is processed by BP filter 122 centered at frequency f.sub.IF that provides a filtered signal V.sub.IF ' to a conventional demodulator 124.
FIG. 2A contains a chart 200 illustrating the spectrum of the conventional image reject down converter 100 of FIG. 1 for a down-converted signal having a relatively high center frequency (f.sub.IF). The gain of passive filter 102 is represented by line 202. Thus, the desired information in the form of a lower side band 204, centered around the receiver frequency f.sub.RF, is not removed by passive filter 102, since line 202 has a high cut-off at the lower f.sub.3 and upper f.sub.4 edges of lower side band 204. However, the unwanted image in the form of an upper side band 206, centered around an image frequency f.sub.IM, is removed by passive filter 102, which has minimal gain (maximum attenuation) at the lower f.sub.1 and upper f.sub.2 edges of sideband 206. The difference between the image frequency (f.sub.IM) and the local oscillator frequency (f.sub.LO) is the same as the difference in frequency between the local oscillator frequency (f.sub.LO) and the reference frequency (f.sub.RF). Summer/subtractor 120 is used to select either upper side band 206 or lower side band 204 to be removed from the received signal as follows: EQU V.sub.IF =V.sub.I +V.sub.Q
(removes upper side band) EQU V.sub.IF =V.sub.I -V.sub.Q
(removes lower side band)
A recent trend has been to implement communication system receivers as integrated devices, sometimes referred to as "on chip" receivers, to reduce their size and manufacturing costs. One approach for implementing receivers on integrated devices is to reduce the center frequency of the down-converted signal so that the demodulators can be integrated at a much smaller current. However, there are several drawbacks with using conventional down converter arrangement 100 for down-converted signals having lower center frequencies. Specifically, as the center frequency of down-converted signals is decreased, the upper and lower side bands 206 and 204 are closer to the local oscillator frequency (f.sub.LO). As the upper and lower side bands 206 and 204 move closer to the local oscillator frequency (f.sub.LO), the image rejection near the image frequencies to be removed can become insufficient to remove the entire image band due to roll-off in passive filter 102.
FIG. 2B contains a chart 210 that illustrates the spectrum of image reject down converter 100 for a down-converted signal having a relatively low center frequency (f.sub.IF), of about 4 times the signal bandwidth. As illustrated by chart 210, upper side band 206 is not filtered in the same manner as in FIG. 2(a). The upper side band is no longer sufficiently removed by passive filter 102 since upper side band 206 is situated in the roll-off region of filter 102. Particularly, the upper edge f.sub.2 and lower edge f.sub.1 side band 206 are not equally filtered by passive filter 102 for down-converted signals having relatively low center frequencies (f.sub.IF). Thus, the upper side band 206 is not completely cut off as in FIG. 2(a), and the partial attenuation which does occur is asymmetric, with the amplitude of the lower edge f.sub.1 and upper edges f.sub.2 being different.
FIG. 3 is a chart 300 illustrating image rejection provided by image reject mixer 107 (indicated by line 302), passive filter 102 (indicated by line 304), BP filter 122 (indicated by line 305), and the total image rejection of image reject mixer 107, passive filter 102, and BP filter 122 (indicated by line 306). The attenuation provided by image reject mixer 107 (line 302) is centered at F.sub.IF, the location of which is represented by point 308. FIG. 3 does not necessarily completely and accurately illustrate the image rejection of image reject down converter 100 and filter 102, and is provided to convey the limitations and asymmetric image rejection characteristics of image reject down converter 100 and filter 102.
Where:
f.sub.RF is the receiver signal frequency, PA1 f.sub.IM is the image frequency band to be removed, PA1 f.sub.IF is the center frequency of the down-mixed signal, PA1 f.sub.1 is the lower edge of image band, and PA1 f.sub.2 is the upper edge of image band
then EQU f.sub.LO =f.sub.RF +f.sub.IF ;
and EQU f.sub.IM =f.sub.LO +f.sub.IF =f.sub.RF +2*f.sub.IF.
As is illustrated by chart 300, image rejection at f.sub.1 is different (asymmetric) from the image rejection at f.sub.2. In addition, the worst case image rejection is often unacceptable for certain applications. For example, the worst case image rejection is typically less than seventy (70) decibels (dB) at f.sub.1 l, which is unsuitable for many communication systems, such as wireless telephony. Therefore, the conventional approach for providing image rejection in signal down converters does not provide adequate image rejection for down-converted signals having relatively low center frequencies (f.sub.IF).
Accordingly, based upon the need for image rejection receiver down converters and the limitations in prior approaches, an approach for performing image rejection in receiver down converters that produces symmetric image rejection at the upper and lower image band edges while providing a worst case image rejection level suitable for wireless communication applications is highly desirable.